Two-Dimensional Stiffness Profiles from SASW Testing at Selected Sites: 1994 Northridge Earthquake

1994年1月17日北岭地震中的几个土壤和岩石场地表现出异常高的地表加速度。 该项目的现场部分基本上是一维的，也就是说，在每个地点只测量小应变材料刚度的垂直剖面（以剪切波速度计）。 未提出对刚度剖面的横向变异性进行调查。 采用一维（1-D）方法是由于该项目的一年时间框架，也是因为安装许多钻孔所涉及的高成本，这些钻孔是使用钻孔地震技术开发二维（2- D）剖面所必需的。 其目的是开发2-D（在一种情况下，3-D）的刚度配置文件。 不采用井中地震技术（如井间测试、井下测试或悬浮测井）。 相反，非侵入SASW技术涉及瑞利型表面波被利用。 该技术是非侵入性的，因为源和接收器都放置在地面上。 面波频散的测量在现场进行。 然后在实验室中使用正演模拟来确定现场的剪切波速度剖面。 除了非侵入性的成本优势外，SASW测试的一个重要优势是它可以用于测量夹在较硬层之间的较软层，这是工程研究中的一个潜在关键测量。 该项目包括在两个地点进行SASW测试（表面波频谱分析）：Tarzana地点和La谢内加与I- 10高速公路交汇处。 在这两种情况下，地面站都测量到了异常高的地面震动。 在每个地点，将使用高能移动的振动器（可控震源）进行现场地震测试。 这种震源加上四个或更多个1赫兹地震检波器的多通道记录，应可使横向尺寸（视地点而定）约为100至300米的剖面发展到深度约为30至60米。 就Tarzana场址而言，将在南北和东西两个方向进行剖面分析，以便绘制地下剪切刚度的三维图像。 这项研究的结果有可能提供一个重要的贡献，以了解地面震动水平在1994年北岭地震，以及开发具有成本效益的2-D和3-D地震剖面预测地面响应在未来的地震。 ***

9416433 Stokoe Several soil and rock sites shaken by the Northridge Earthquake of January 17, 1994 exhibited unusually high ground surface accelerations. The insitu portion of the project is basically one dimensional; that is, only a vertical profile of small-strain material stiffness, in terms of shear wave velocity, will be measured at each site. No investigation of the lateral variability of the stiffness profiles is proposed. The one -dimensional (1-D) approach is taken due to the one year time frame for the project, also because of the high cost involved in installing numerous boreholes that would be necessary to develop two-dimensional (2- D) profiles using borehole seismic techniques. The objective is to develop 2-D (and in one case, 3-D) stiffness profiles. Borehole seismic techniques (such as crosshole testing, downhole testing or suspension logging) will not be employed. Instead, the nonintrusive SASW technique involving Rayleigh-type surface waves is utilized. The technique is nonintrusive because both the source and receivers are placed on the ground surface. Measurement of surface wave dispersion is performed in the field. Forward modeling is then used in the laboratory to determine the shear wave velocity profile at the site. In addition to the cost benefit of being nonintrusive, an important benefit of SASW testing is that it can be used to measure softer layers sandwiched between stiffer layers, a potentially critical measurement in engineering studies. This project involves SASW testing (spectral analysis of surface waves) at two sites: The Tarzana site and the La Cienega and I- 10 highway interchange. In both cases, unusually high ground shaking was measured with surface stations. At each site, in situ seismic testing will be performed using a high-energy mobile vibrator (Vibroseis). This source, coupled with multi-channel recording of four or more 1-Hz geophones, should permit profiles on the order of 100 to 300 m in lateral dimension (depending on the site) to be developed to a depth on the order of 30 to 60 m. In the case of the Tarzana site, profiling will be performed in both the north-south and east-west directions, allowing a 3-D image of the underground shear stiffness to be developed. The results of this study have the potential to offer an important contribution to the understanding of ground shaking levels generated during the 1994 Northridge Earthquake as well as to the development of cost-effective 2-D and 3-D seismic profiling for the prediction of ground response during future earthquakes. ***